Aspects of the present invention are directed to slider fabrication, and in particular to an improved manner of measuring flatness of a slider surface.
Sliders are fabricated for utilization within data storage disk drives (e.g., hard disk drives) for positioning a magnetic head including read/write elements relative to one or more spinning disks. Each slider typically includes read/write elements along with electrical contacts to facilitate electrical connection with an electronic data control system. Sliders are also provided with air bearing features that controllably affect the manner by which the slider “flies” at a fly height on an aerodynamic air bearing created by a spinning disk, forming a head-media separation distance between the slider and the disk. Specifically, the aerodynamic and topographical (height-related) properties of the slider and a slider surface thereof influence the fly height in addition to pitch, roll, head-media separation distance, and other important features of the data storage disk drive. The aforementioned features range in scale from nanometer to millimeter size.
The density of data tracks on disk surfaces has been increasing in order to obtain greater data storage within a given disk surface area. This surface storage density is commonly referred to as areal density. Specifically, the linear density of the data tracks themselves has increased and the data tracks have become narrower. The data tracks have become more tightly packed and the radial spacing between data tracks has decreased in order to increase areal density. In order to continue to improve the areal density and to improve read/write performance of the data storage disk drive, it is desirable to reduce the head-media separation distance between the read/write elements on the slider and the disk. A smaller head-media separation distance may increase the magnetic recording density at least in part by improving an associated signal-to-noise ratio and associated read/write precision of the magnetic head vis-à-vis the media (e.g., the disk). Typically, a head-media separation distance between the slider and a spinning disk is 10 nm or less.
An ideal slider for use in a hard disk drive would have flat and uniform surfaces, including the air bearing surface. However, a slider is generally not merely flat, but has a purposely-contoured air bearing surface in order to permit function of the air bearing, leading to a fly height, as discussed above. Flat surfaces would generally lead to more predictability and precision, especially on a very small, nanometer scale. In practice, however, sliders generally do not have desired, ideal, or uniform flatness (defined by flatness parameters), and may have surface feature imperfections for a variety of reasons. One way to increase performance and to reduce the head-media separation distance is to better flatten or reduce the roughness (and thus increasing the flatness) of certain of the slider surfaces, in particular an air bearing surface. The air bearing surface generally faces the media or disk over which the slider would eventually fly over during use. Better flattening and/or reduction in roughness of the slider surfaces may lead to more uniformly flattened or contoured slider surfaces, which may be desirable.
One method by which slider surfaces can be flattened and/or reduced in roughness is through lapping. Lapping, as used herein, is a machining process related to polishing by which two surfaces are rubbed together with an abrasive between them, whereby unwanted material is removed from an object, such as a slider, in order to give desired surface qualities, dimensions, and/or shape of the object, such as flatness. Lapping, also referred to as plate lapping, can operate on the nanometer scale, and can preferably have a high degree of precision for materials processing. Lapping, as used herein, may utilize a lapping plate, and both free abrasive lapping and fixed abrasive lapping are contemplated. A slider may have undesirable imperfections before lapping, but may also retain some degree of imperfection during, and after a lapping process.
Multiple sliders are often fabricated from a single wafer that is cut into slider bars. Slider bars may be held by bar carriers during lapping. The wafer may be formed of the materials and layers specified for a desired slider construction. The wafer may be composed of various materials, including ceramic materials and/or silicon. Examples include Al2O3, TiC, etc. From such a wafer, a portion of the wafer is separated from the remainder, the portion is typically rectangular and is dimensioned based upon a desired number of rows and number of sliders in each row. Known processes utilize a rectangular wafer portion that is appropriately sized to produce roughly 40 rows of sliders with each row providing about 60-75 sliders. The wafer portion is then diced into slider bars equal to the number of rows provided, creating an equal number of slider bars as there are initially rows.
After separation into the slider bars, it may be determined that a surface of at least one of the slider bars has imperfections. If at least some imperfections are found, such as surface roughness, the slider bars may benefit from flattening. The surface may then be lapped in order to increase surface flatness and to decrease surface roughness. An example slider bar may have dimensions of one slider length wide by 70 slider widths long. An example surface subject to a lapping step is a common air bearing surface of one or more slider bars. To assist lapping, carriers, such as slider bar carriers, can be configured to support each of the slider bars that have been separated from a wafer and to accurately hold each of the slider bars at a proper orientation for lapping. As above, a typical slider bar carrier may hold 40 slider bars each of a length of 70 sliders to make, for example, a total of 2800 sliders. The air bearing surface of each slider is preferably lapped to comply with desired slider surface characteristics, such as slider curvature metrics (degree of flatness) in terms of crown, cross, and/or twist curvature. A slider bar carrier, as used herein, may be a free-bar carrier or a production bar carrier.
A typical slider is roughly rectangular when viewed from an air bearing surface of the slider, the slider having a length and a width. As used herein, three types of across-slider curvature metrics are defined, as follows. Crown curvature is the curvature of a slider following the length (longitudinal axis) of a slider. Cross, camber, or cross-crown curvature is the curvature of a slider following the width (transverse axis) of a slider. Twist curvature is curvature where a slider has at least a degree of twist about its longitudinal or transverse axes, that is, where one end of a slider is not coplanar with another end of the same slider. While twist does not typically directly affect fly height, high twist curvature values, either positive or negative, can cause other negative tribological (i.e., interactive surfaces in relative motion) effects, such as increased wear due to changes in friction or lubrication, and/or induced roll.
After a lapping process or step is performed, individual sliders of the slider bars may be separated from one another, e.g., by dicing, for assembly and use in a storage disk drive head suspension assembly. However, slider surface flatness may preferably be verified prior to dicing the sliders to be sure that the air bearing surface of each slider is flattened to a desirable specification for usage. If the slider surface flatness is determined to be suboptimal, a further lapping step could be performed prior to dicing the sliders from the slider bars. It may be undesirable or inefficient to perform a lapping step on each slider individually.
Slider surfaces are typically inspected using optical metrology technology such as utilizing commercially-available interferometric systems. An interferometer is a known device that utilizes interferometry, where electromagnetic waves are superimposed in order to extract information about the waves. Interferometers are typically used to measure small displacements, refractive index changes, and surface irregularities. For the purposes of this disclosure, an interferometer may be used to measure surface irregularities of one or more sliders.
Various interferometer systems, such as small aperture interferometers, are commercially available from, for example, Bruker Corporation, Zygo, or Taylor-Hobson, specifically including a small aperture optical interferometers having approximately a 1-2 mm imaging area field-of-view (FOV) that is sized to image and model flatness of individual sliders. Small aperture interferometers would typically image an individual slider at a time while the sliders are still present in slider bar form.
Various slider fabrication process steps include a high level of complexity, low tolerances, and small size specifications. Typical process steps include fine line photolithography, reactive ion etching, ion milling, and/or thin film deposition. A lapping step, as stated, is often used to ensure that the surface roughness of the slider air bearing surface of is minimal. Materials used in the manufacturing of a slider vary depending on the desired properties. Typically, magnetic heads, which may include read/write elements, are constructed from a variety of materials, such as magnetic alloys, metal conductors, ceramic, and polymer insulators in a complex three-dimensional (3D) structure having precise tolerances.